Follistatin was first identified in bovine and porcine follicular fluid by its ability to suppress FSH secretion from pituitary cell cultures. Follistatin is a glycosylated single-chain protein that occurs in at least 6 forms ranging from 31- to 39-kDa molecular mass; smaller forms represent a carboxyl terminal truncation of the larger precursor. Although follistatin is known to be expressed in a number of tissues, a major site of production is the granulosa cells of ovarian follicles. In cattle, follistatin mRNA expression increases as follicle size increases; the strongest signal was observed in preovulatory follicles. flovent inhaler
Based on data obtained in vitro, it has been postulated that follistatin modulates granulosa cell function in an autocrine manner and that its actions favor luteinization or atresia. Folli-statin is a specific binding protein for activin, and its actions are believed to be mediated mainly through neutralizing the effects of activin. Follistatin has been measured in peripheral blood of humans and sheep with the supposition that it might act as a circulating hormone; however, concentrations of follistatin varied little during the estrous cycle of the ewe, and there was little or no reduction after ovariectomy. Although the regulation, production, and actions of follistatin have been studied extensively in vitro, data on its role in ovarian function in vivo are insufficient and confusing.